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STOCK-BASED EMPLOYEE COMPENSATION (Restricted Stock Units and Performance Share Units) (Details) - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock Units and Performance Share Units      
Share-Based Awards - Shares      
Unvested at December 31, 2017 524,696    
Granted (in shares) 153,637    
Vested (in shares) (184,648)    
Performance Share Units Adjustment (in shares) 12,676    
Forfeited (in shares) (21,685)    
Unvested at June 30, 2018 484,676 524,696  
Share-Based Awards - Weighted Average Grant Date Fair Value      
Unvested at December 31, 2017 $ 1,431.88    
Granted (in dollars per share) 2,035.68    
Vested (in dollars per share) 1,277.74    
Performance Share Units Adjustment (in dollars per share) 1,587.94    
Forfeited (in shares) 1,649.68    
Unvested at June 30, 2018 $ 1,676.34 $ 1,431.88  
Total unrecognized estimated compensation expense, unvested share-based awards $ 496.1    
Total future compensation cost related to unvested share-based awards, expected period of recognition 2 years 1 month    
Restricted Stock Units (RSUs)      
Share-Based Awards - Shares      
Granted (in shares) 103,916    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share) $ 2,036.58    
Vesting period (in years) 3 years    
Grant date fair value $ 211.6    
Performance Share Units 2018 Grants      
Share-Based Awards - Shares      
Granted (in shares) 49,721    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share) $ 2,033.79    
Grant date fair value $ 101.1    
Estimated number of probable shares to be issued (in shares) 49,475    
Maximum shares that could be issued (in shares) 98,950    
Minimum shares that could be issued (in shares) 38,165    
Performance Share Units 2017 Grants      
Share-Based Awards - Shares      
Granted (in shares)   73,893  
Unvested at June 30, 2018 64,769    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share)   $ 1,735.10  
Grant date fair value   $ 128.2  
Estimated number of probable shares to be issued (in shares) 84,507    
Maximum shares that could be issued (in shares) 129,538    
Minimum shares that could be issued (in shares) 51,512    
Performance Share Units 2016 Grants      
Share-Based Awards - Shares      
Granted (in shares)     85,735
Unvested at June 30, 2018 66,270    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share)     $ 1,302.25
Grant date fair value     $ 111.7
Estimated number of probable shares to be issued (in shares) 111,196    
Maximum shares that could be issued (in shares) 148,953    
Minimum shares that could be issued (in shares) 39,498